Human immunodeficiency virus (HIV-1) infection is characterized by an inexorable decline in CD4 + T cells. CD4+ T cells have been shown play an important role in maintaining memory cytotoxic T cell (CTL) responses in a number of viral infections. Recently, in mouse models, it has been demonstrated that CD4 + T cells help CTL responses via CD40 ligand-CD40 interactions. The project examines the role of CD4+ T cells and CD40 ligand in producing an effective memory cytotoxic T cell (CTL) response in both normal human volunteers and HIV-1 infected individuals. We have demonstrated that CD4 help is required for optimal in-vitro CTL responses against influenza in 2/2 HIV-1 uninfected subjects. The addition of CD40-Ligand dramatically enhanced CTL responses and was able to completely compensate for CD4 help in CD4- depleted conditions in 2/2 HIV-1 uninfected subjects. In HIV-1 infected individuals, CD4 help was required for optimal HIV-1-specifc in-vitro CTL responses in 5/6 patients. In one long term non-progressor patient, optimal CTL responses could be induced in-vitro without CD4 help. CD40- ligand could enhance CTL responses in 5/6 HIV-1 infected patients, however, CD40 ligand could not completely replace CD4 help in 3/6 patients. The addition of exogenous cytokines such as IL-2, IL-12 or IL-15 was able to substitute for CD4 T cell help in most patients. Thus our findings demonstrate the importance of CD4 help and CD40 ligand in inducing effective memory anti-viral CTL responses. Our inability to rescue CD4 help with CD40 ligand in 3/6 HIV-1 infected patients suggests an intrinsic defect of CD8 cells in these patients that will warrant further study. - CD4 help; cytotoxic T cell response; human immunodeficiency virus-1; human tissues; CD40 ligand; CD8 tetramer.